1. Field of the Invention
The present invention relates to an optical receiving apparatus having a simplified circuit that generates soft-decision data for use in error correction.
2. Description of the Related Art
To achieve increase in a transmission distance and a capacity of a wavelength division multiplexing (WDM) optical communication system, it is important to secure a signal quality against degradation of optical signal-to-noise ratio (SNR). When an optical communication system of 10 gigabits per second (Gbps) in each wavelength is enhanced to have a rate of 100 Gbps, an improvement in an optical SNR resistance of about 10 dB is required to obtain the same signal quality.
As a method to improve an optical SNR resistance, error correction techniques are employed (For example, “Next generation FEC for optical communication” by T. Mizuochi et al., OFC/NFOEC 2008, OTuE4, 2008). For the purpose of obtaining a high correction capability applicable to optical communication systems having a rate on the order of 100 Gbps, error correction techniques that are based on soft decision have been studied (for example, Japanese Patent Application Laid-open No. 2005-33547, “Next generation FEC for optical communication”, and “Proposal for frame structure of optical channel transport unit employing LDPC codes for 100 Gb/s FEC” by Y. Miyata et al., OFC/NFOEC 2009, NThB2, 2009).
In a digital-coherent optical receiving apparatus as disclosed in the above literature by Y. Miyata et al., a bit resolution of an analog-to-digital (A/D) converting circuit that converts an analog value of a received optical signal to a digital value, or a data bit width in a digital-signal processing circuit such as a received-signal demodulating circuit, which is connected at a later stage of the A/D converting circuit, is typically sufficiently larger than a bit width of soft-decision data necessary for error correction. In order to realize a soft-decision error-correcting circuit having a throughput of around 100 Gbps, realization of reduction with a bit width of soft-decision data in a simpler configuration is important in terms of reduction in a circuit size.
In a practical optical communication system, distribution of noise contained in a received optical signal can be symmetrical or asymmetrical depending on modulation methods applied. Therefore, it is important for practical use of a system to easily obtain optimal soft-decision data regardless of how noise contained in a received optical signal is distributed.
Soft-decision information can be generated by directly implementing in a digital circuit a comparison with multiple soft-decision thresholds as disclosed in Japanese Patent Application Laid-open No. 2005-33547, for example. However, reduction of the comparison operation is important for decreasing the circuit size.
A bit-width reduction technique based on reliability information is disclosed in Japanese Patent Application Laid-open No. 2009-27470 as a soft-decision-data generating circuit that reduces a bit width. When considering signal processing at a rate of around 100 Gbps, however, it is desirable to have a method that can more easily extract reliability information to generate soft-decision data.
Other techniques have also been disclosed: a technique that selects only bit data on a most significant bit (MSB) side in soft-decision data and discards the remaining bit data on a least significant bit (LSB) side (for example, Japanese Patent Application Laid-open No. 2003-258758), a technique that shifts a position where bit data is selected according to conditions of a transmission channel (for example, Japanese Patent Application Laid-open No. 2003-258762), and a technique that selects a soft-decision threshold based on hard-decision data for an immediately preceding bit (for example, Japanese Patent Application Laid-open No. 2004-349888). However, none of these techniques disclose a specific method of easily generating, corresponding to noise distribution of a received optical signal, soft-decision data with a reduced bit width from a digital signal output by a decoding circuit.
The conventional techniques have following problems.
With respect to the conventional soft-decision-data generating circuit that is based on data comparison with multiple soft-decision thresholds, the comparison operation for generating soft-decision data increases when the bit width of data being handled increases.
The conventional soft-decision-data generating circuit that reduces a bit width according to reliability information requires an operation for generating soft-decision data. Therefore, high processing load is required and the circuit size is increased.
Furthermore, there is a problem in that soft-decision data cannot be easily generated with a reduced bit width from a digital signal output by a decoding circuit as appropriate according to noise distribution of a received optical signal.